A storage apparatus has a plurality of hard disk drives (hereinafter, referred to as HDD) and manages the group of HDDs by using a system known as RAID (Redundant Array of Independent/Inexpensive Disks). The group of HDDs stores various kinds of data. It may store the data that is not accessed for a certain time period, for example, an old file or backup data. The group of HDDs that forms a RAID GROUP (hereinafter, referred to as RAID group) that stores the data is in an unused state during the period when the data is not accessed.
As a method for reducing power consumption of a storage apparatus by changing the operating mode of apparatus to a power saving mode, a method of stopping an unused HDD among a plurality of HDDs that form the storage apparatus or otherwise methods are known. For example, PTL 1 discloses a technique of reducing the power consumption by monitoring I/O to the storage apparatus and changing the operating mode of the HDDs that has not been accessed for a certain time period to a power saving mode.
Incidentally, as for the storage apparatus, it is required to decide a storage capacity to be needed in future and allocate it to a volume in advance in the design stage. For that reason, HDDs that are not used in fact have to be bought in advance; accordingly, the cost the storage user bears tends to increase. In order to solve the problem, PTL 2 discloses a Thin Provisioning technique that provides a virtual volume, which is a virtual storage area, for a host computer and allocates a necessary capacity when data is written from the host computer in fact.
In the technique, an area called pool area is prepared on a storage system so that a plurality of host computers shares a plurality of virtual volumes via the pool area, and when data is to be written from a host computer, an area required for writing the data is allocated to the virtual volume. The storage system that adopts the technique is capable of flexibly increasing the capacity of the pool area by adding an HDD as required.